@whale-av The 3.5mm jack without a resistor would actually add more noise than with it,. The input jack is a switchting type jack, with nothing plugged in the tip and ring is shorted directly to ground, if you just plugged in a bare plug, it would break that short to ground, the plug would become an antenna and while being very short and limited in the wavelengths it could pickup, it would pickup a good deal and certainly more than the Johnson noise of a resistor, if you just plugged in a cable and left the other end unplugged, you would get even more since your antenna is now longer and can pick up longer wavelengths better. A resistor added in, will just resist, those weak RF signals will need to over come that resistance to reach the preamp and be amplified. Johnson noise is a very small factor, it does contribute, but it is not something one would really want to try and exploit as a noise source, a few feet of and wire will give you considerably more. A rather simplified explanation and not completely correct, consider it practical but not technical.
As an aside, the second ring on the standard tip, ring, ring 3.5 mm plug that we see on phones and anything that can take a headset is powered, those headsets use electret microphones which need some voltage to function. I am not sure what this voltage is, but if you can find a zener diode with a reverse breakdown voltage that is less than the voltage supplied by the jack for a microphone, you could likely build a noise generator into a standard 3.5mm plug with little issue. Zener diodes are generally thought of as poor noise generators, their output level is quite erratic, they are too random to be good noise, but that is great when your needs are random and not pure white noise. There is no real gain to building such a noise source into a plug, just plugging in any cable and leaving the other end floating will do just as well and with less effort.
